1. Field of the Invention
This invention relates to zoom lenses for photographic purposes, more particularly but not exclusively, for video movie cameras
2. Prior Art
U.S. Patent Specification No. 3,524,696 which draws priority from a Japanese Patent application filed in 1967, discloses a zoom lens including twelve elements arranged in four units. There is a positive unit facing the long conjugate and movable for focussing. There is a negative unit movable for zooming. There is a second positive unit which is movable for zooming and there is a rear unit which is stationary. The positive unit facing the long conjugate consists of a cemented doublet and a single element therebehind. The negative unit consists of a negative element and a negative doublet concave towards each other. The second positive unit consists of a single component including at least one positive element.
In U.S. Pat. No. 3,524,696, the fourth unit consists of a positive element, a negative element and a doublet consisting of a negative element and a positive element.
The zoom lens described in U.S. Pat. No. 3,524,696 has a zoom range of only 2.times. and a f-number of only 3.5.
In the ensuing twenty years there have been several proposals for zoom lenses in which the first three units are similar to those of U.S. Pat. No. 3,524,696 and with differing rear units. For example, in U.S. Patent Specification No. 4,105,291 the rear unit includes six elements which are positive, positive, negative, positive, positive and negative, in power, respectively. The zoom lens of U.S. Pat. No. 4,105,291 has a zoom range of only 2.8 and a f-number smaller than 1.0.
In U.S. Patent Specification No. 4,096,586, the behind the rearmost moving unit there are two units including six elements of positive, positive, negative, positive, negative and positive powers, respectively. That known zoom lens has a zoom ratio of 2.7 and a f-number of 1.4.
In U.S. Patent Specification No. 4,368,954, there is disclosed a zoom lens having a rear unit including five elements which have positive, negative, positive, positive, negative powers, respectively. That zoom lens has a zoom ratio of about 4.7 and a f-number of 1.26.
In U.S. Patent Specification No. 4,380,377, the zoom lens there disclosed has four elements behind the rearmost moving unit, divided into two spaced sub-units. The elements have positive, negative, negative and positive powers, respectively. That zoom lens has a zoom range of about 2.9 and a f-number of 1.26.
In U.S. Patent Specification No. 4,456,341, the zoom lens therein disclosed has five elements behind the rearmost moving unit, divided into two spaced sub-units. Those five elements have positive, positive, negative, negative and positive powers, respectively. That zoom lens has a zoom ratio of 2.1 and a f-number of 1.4.
In U.S. Patent Specification No. 4,629,292, the zoom lens therein disclosed has four elements in the rear unit and they have positive, negative, positive, negative powers, respectively. That zoom lens has a zoom ratio of 2.9 and a f-number of 1.4.
It is an object of the present invention to provide a zoom lens having in combination a higher zoom ratio, superior f-number and fewer elements than the zoom lenses of the prior art.